Conventionally, an image forming apparatus such as a printer, a copier, a facsimile machine, or a multifunction machine, for example an electrophotographic color printer is provided with image forming units of multiple colors, and in each of the individual image forming units, a toner image of each color is formed on the surface of a photosensitive drum, the toner image is sequentially transferred onto a sheet by a transfer roller to form a color toner image, and the color toner image is fused to the sheet in a fuser.
By the way, because the individual image forming units are formed in the same shape due to functional, structural, and costwise factors, they need to be attached to the positions assigned by the toner color in the printer main body, that is, the apparatus main body.
Then, an engaging member and an engaged member are arranged on each of the image forming units and the apparatus main body, respectively, and if the engaging member and the engaged member do not match, the image forming unit cannot be attached to the apparatus main body (e.g., see Patent Document 1).